Abstract magneto-resistance. It is therefore a little surprising

Abstract

 

The resonant inelastic x-ray scattering (RIXS) of MnO, Mn2O3, and MnO2 are measured with high energy resolution at a third generation synchrotron radiation source. The spectra across the Mn L2,3 absorption edges turn out to be rich in spectral features. The spectra are dominated by the low energy d-d and C-T excitations, both dipole-allowed in the RIXS process. The spectra show a variety of fine structure varying in shape and intensity in the three oxides of Mn. A significant dependence of these low-energy excitations is observed on the valence of the Mn cation/crystal structure of the compounds. As far as Mn2O3 and MnO2 are concerned these are the first ever RIXS studies to be reported on the Mn L edge in them.

 

Keywords:

Synchrotron radiation source

 Resonant inelastic x-ray scattering

Mn L edge

Low energy d-d and C-T excitations

 

Introduction

 

Coming of third generation of high energy resolution and greater brightness synchrotron radiation sources has made it possible to delineate low energy loss peaks from the resonant inelastic x-ray scattering (RIXS) spectra. RIXS is rapidly gaining in importance in study of low energy excitations in solids 1. As is well known the d-d and charge-transfer C-T excitations, forbidden in all photoemission and x-ray absorption, become dipole-allowed in the RIXS mechanism 2. The technique is being particularly employed in study of the low energy excitations in the transition metal compounds for example, 3-10. Although the potentiality of the L3 RIXS was indicated a little earlier 11, 12 its use is becoming popular only now after the advent of the new synchrotron radiation sources enabling higher energy resolution and greater brightness.

 

Oxides of Mn carry a special importance in view of the large number of Mn-based compounds exhibiting colossal magneto-resistance. It is therefore a little surprising that the oxides of Mn have not received as much attention as they ought to have. Of the three oxides of Mn the MnO has received some attention but as far as the others- Mn2O3 and MnO2 – are concerned this may be the first ever RIXS study to be carried out on the Mn L edge in them. We apply the resonant inelastic scattering technique in study of d-d and C-T excitations in case of the three oxides of Mn. These low energy (of the order of ~1 eV) play an important role in determining the behavior of the system at even very low thermal energies. The antiferromagnetic ordering in MnO is related to the ligand field 13. Various core-hole spectroscopies are employed to obtain knowledge about the ground state, but the presence of a core hole complicates the analysis of the data. The d-d excitations in transition metal compounds are forbidden by the dipole selection rules and therefore appear very faint in optical spectroscopy. These selection rules are relaxed at low electron energies and hence can be studies with electron energy loss spectroscopy (EELS), but EELS measurements tend to be very surface sensitive.

 

The resonant inelastic x-ray scattering (RIXS) does not suffer from such limitations. It can be represented by two incoherent steps, x-ray absorption followed by x-ray emission, adequately. The first step produces a core-hole states in the L2,3 x-ray absorption spectrum. The second step comprises of decay of the intermediate state, either to the ground state yielding the electronic recombination peak or to a different state of the electronic multiplet without a core hole- resulting in the energy loss structures. As the process involves two dipole transitions, the final state has the same parity as the initial state, and d-d excitations are not forbidden. Aside from the d-d excitations, one can also observe charge-transfer excitations. These are transitions that involve the transfer of an electron from the ligand O2- band to the metal ion.

 

In this paper we report the results of our study of the oxides of Mn employing RIXS, X-ray absorption and normal (non-resonant) x-ray emission spectroscopies.

 

Experimental

The X-ray emission spectroscopy (XES) measurements were performed at Beamline 8.0.1 of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL)15. The endstation uses a Rowland circle geometry X-ray spectrometer with spherical gratings and an area sensitive multichannel plate detector (MCP). The oxygen K emission line (probing the 1s ? 2p transition) was measured for all oxides,. The oxygen K XES spectra were excited near the oxygen 1s ionization threshold, at 540.8 eV, to suppress the high-energy satellite structure. The spectrometer resolving power (E/?E) for emission measurements was about 800.

 

Results & Discussion

 

       

 

Figure 1. (left) X-ray absorption spectra of MnO, Mn2O3, and MnO2. The vertical arrows indicate the excitation energies chosen for the RIXS spectra. (right) The figure shows the corresponding normal x-ray emission spectra for the three oxides.

   

Figure 1(a) above shows the L2,3 x-ray absorption spectra of the three oxides of Mn viz. MnO, Mn2O3, and MnO2. The MnO spectrum closely resembles that of Butorin et al 3, the Mn2O3 and MnO2 those of Shota Kobayashi et al. 14.The Mn L3 peak in MnO shows the characteristic forked structure. The main MnL3 and MnL2 peaks in the spectra both shift to higher energy side as the Mn valence increases from 2+ in MnO to 4+ in MnO2 which is only to be expected. Figure 1(b) shows the corresponding normal (non-resonant) x-ray emission spectra for the three oxides. Here also the main peaks show a shift to the higher energy side as the valence of the Mn ion.

 

Figure 2 (a) below shows the resonant RIXS spectra in MnO for the excitation energies corresponding to those represented by the vertical bars. These excitation energies are also marked by arrows in Fig. 1(a). The spectra in Fig. 2(a) are shown plotted on an energy-loss scale in Fig. 2(b) at the right. The RIXS spectra resemble those of Butorin at al 3 and Ghiringhelli et al 9 in the sense that fine structure peaks are observed on the lower energy side of the elastically scattered peak marked by the vertical line in the figure on right but also differ from them in some other respects. Our spectra show more peaks than those observed by Butorin et al 3, due probably to better resolution and intensity in our experiment. In this regard, our spectra are more like those observed by Ghiringhelli et al 9 but differ from them in respect of the relative intensities of the peaks.

 

The observed peaks arise due to one of the four reasons: (i) the elastically scattered or recombination peak, (ii) the low energy d-d excitations, (iii) the charge transfer C-T excitations, and (iv) the normal (non-resonant) L?,? x-ray emission lines. The elastically

                                   

 

Figure 2. (left) The resonant (RIXS) spectra for MnO (on the left). The short vertical bars indicate the excitation energy in each case. (right) The RIXS spectra are replotted on an energy-loss scale.

scattered peak appears at the same energy as the excitation energy, except for possibility of photon losses. The strength of this peak shows that the excited electron stays bound to the the core hole as an exciton. It is also seen in the spectra atht the relative intensity of the recombination peak decreases with the increasing excitation energy. This is due to spin-ordering of the excited states. Alternatively, this may be due to the fact that if absorpyion is weak, the matrix element from the excited to the ground state is also small, and the core hole is more likely to decay to an excited final state 3.

 

Turning our attention to the other peaks we see that the emission spectra show a strong dependence on the excitation energy. The spectra consist of both resonant and non-resonant parts which can be identified. The resonant part follows the excitation energy due to energy conservation. The emitted photon energy Eph must increase with the increasing incident photon energy Einc in accordance with the equation Einc + Eg = Eph + Ef, wherein Eg and Ef are the energies of the ground and the final states in the emission process. This fact gives rise to energy-loss resonant features that occur at a constant energy below that of the elastically scattered peak. The nonresistant features, on the other hand, appear at the constant energies of the emitted photons. As established by Butorin et al3, the peaks in the region from 0 to about -6 eV represent the low-energy d-d excitations. A resolved fine structure in the energy-loss spectra can be clearly seen in all the spectra p to u. The fine structure broad peaks appearing at energies < -6 eV are the charge transfer C-T peaks. These weak and broad structures become more and more visible at increasing excitation energies (as we go from spectra p to spectra in u). Butorin et al 3 have reproduced the fine structures in the spectra employing the spherical approximation model. Ghirighelli et al 9 have, on the other hand employed the single impurity Anderson model and the single ion crystal field model to account for the fine structures in their observed spectra. Both models are shown to reproduce well the d-d excitations, but the Anderson model also satisfactorily accounts for the C-T excitations.   Figure 3 shows the RIXS spectra on energy-loss scale for the two other oxides of Mn viz. Mn2O3 (on the left) and MnO2 (on the right). Comparing these with the RIXS spectra for MnO in fig. 1 it is easy to see that the fine structures in the spectra tend to decrease in the order MnO to Mn2O3 to MnO2. Now, the three oxides have different crystal structures and different symmetries. It is therefore tempting to ascribe the decreasing fine structures to the decreasing symmetry in the three oxides. MnO is cubic and has maximum symmetry, Mn2O3 has an orthorhombic structure and has lower symmetry than that in MnO, and MnO2 being monoclinic has the lowest symmetry of the three. Also, MnO2 is always deficient in oxygen which would further decrease its symmetry. As in MnO the structures within -5 eV pertain to d-d excitations and it is respect of these that the differences in the spectra are more pronounced. Basically, only three peaks in Mn2O3 and MnO2 would correspond to the d-d excitations compared to four or five in the MnO. As far as the C-T excitations beyond -5 eV are concerned they are very similar in all the three oxides.     Figure 3. The RIXS spectra for Mn2O3 (on the left) and MnO2 (on the right) plotted on an energy-loss scale.   In conclusion, we demonstrate the potential of Resonant Inelastic X-ray Scattering method to study low energy d-d and C-T excitations employing it in study of the three oxides of Mn. To the best of our knowledge, this is the first ever RIXS study on the Mn L2,3 states in the trivalent Mn2O3 and  tetravalent MnO2 systems. It is observed that the amount of fine structures in the d-d excitation region tends to decrease as one goes from MnO to Mn2O3 to MnO2. This may, amongst other reasons, be due to the decreasing crystal structure symmetry in that order